Sure thing. I would argue that objectively considering a language involves delving in and having a shot, to be honest, but hey.
Smalltalk has been covered by MarkSmith and about the only other "con" I can think of is the relative slowness of the extra indirection caused by message passing rather than direct function calling.
Eiffel is possibly *the* reference in Design by Contract, (which, unsurprisingly, is something Bertrand Meyer cares deeply about). Other pros of Eiffel are the way it handles optimisation (i.e. automatically, with no programmer intervention needed or allowed). On the downside, it's overly verbose, and, of course, not very well known.
Self is interesting in that it's a prototype-based OO language (rather than class systems, one clones objects and behaviours). The standard implementation of Self comes with the "Morphic" user interface, which is somewhat reminiscent of Squeak Smalltalk's UI, but which can accomodate multiple users (sharing of windows between workstations, collaborative editing, and all that jazz). Downside - well, it's uncommon, and you probably won't be able to convince your boss to move over to it.
Dylan is a functional programming language derived from Lisp (and CLOS), and thus has many of the same benefits and downsides. It uses a more "standard" syntax than Lisp or Scheme, which is less intimidating to beginners coming from a C/Java/etc background. The limited number of implementations mean you don't have a great deal of choice with regards to compilers.
CLOS is not a language as such, rather an extension to Common Lisp that provides object orientation (Common Lisp Object System, natch). It's a class-based OO system, so won't scare C++/Java users *too* much (although the fact that methods don't belong to classes might boggle a few minds). It uses multiple dispatch, which makes many relatively common tasks trivially easy to carry out compared to single dispatch languages like C++ or Java, where a significant amount of framework code is required to emulate DD). Downsides - well, it's in Lisp, innit
All of the above (or equivalents) should be pretty much "required reading" for anyone interested in OO, and who thinks, coming from a C++ or Java background, that they know what OO is all about. Apart from Smalltalk and CLOS, I wouldn't necessarily recommend them for regular software development, they are mostly too esoteric and / or too slow. Travel opens the mind, though.
Inform (in all its incarnations) is a massively interesting product, a truly literate programming language designed for producing interactive fiction (or "adventure games" as we used to call 'em). Under the hood there's a full-on prototype based OO language, but Inform 7 adds true "english language" programming built into a very usable IDE. What do I mean by "english language" programming? Here's the start of one of the example programs from the inform website. Yes, this is source code, believe it or not :
"Glass" by Emily Short.
The story genre is "Fairy Tale". The story headline is "A fractured fairy tale". The story description is "The Prince sits awkwardly on the couch, holding his glass slipper and trying to keep it from crushing. Lucinda and Theodora have the ends of the same couch, and they are taking turns seeing who can bend lowest and show off the most cleavage; while the old lady, in her wing chair, carries on about nonsense...
Glass is a conversation-oriented fairy tale, taking place in one room. It was written to demonstrate one approach to handling conversation in Inform 7.
Features a variety of additional verbs, non-player characters with an agenda, and narrative with multiple endings." The story creation year is 2006.
Release along with a solution, source text, a website, an interpreter, cover art, a file of "Walkthrough" called "solution.txt", a file of "Making of..." called "Overview.html".
Use no scoring, the serial comma, and American dialect.
The downside, of course, is that inform is tailored to one specific task, and it's not a great deal of use for anything else. But hell, it's an eye-opener.
Ruby. Ah, Ruby. It's massively trendy, and if you know Ruby (and the obligatory Rails framework), you're gonna be able to get paid a lot of money. But it's more than just a trend, it's a full-on hardcore scripting language. I use it for a lot of things (I've written assemblers and signal analysis software using it, for example). What are its upsides? Well, for starters, it's mainly OO (there are a few non-objectified things, but mostly it's a pretty classic class-based, message passing OO system. You can extend or redefine classes on the fly, even the lowest level ones, which means an end to adding levels of pointless class hierarchy because someone forgot to define method X in some base class. It's also got a certain amount of "functional programming" roots (functions are first class objects and can be passed around, for example) and the popularity of Ruby is possibly part of the reason that ObjC got blocks (and C++0x is getting them). "Blocks", of course, being about as close to first class closures that you can get. There's an enormous amount of useful code out there for Ruby, and bolting together other people's code to make something useful is fast and doesn't involve much hair-tearing compared to combining class libraries in C++. It's also terse. I write probably 1/10 of the number of lines of code to do something in Ruby than I would to do something equivalent in C++ or Java. That's a huge win in terms of productivity. On the downside, it's slow. Really ******* slow. The 1.8 MRI runtime was totally braindead slow, 1.9 is better but it's still like molasses compared to pretty much anything else. There are better implementations out there, but they are still slow.
Common Lisp. Apart from the aforementioned CLOS, it has a few real benefits, most of which are to do with the sheer amount of stuff it can do "out of the box". Most people only see the downsides, though, which are usually given as "eeeagh, bracket hell". It's odd really, it's only syntax, and in most cases Lisp and Scheme use less brackets than Java or C++. Sure, those languages are more varied with their brackets, they use [