BBCode: (Input format / filter.) I love this module. Most of my intended audience is familiar with BBSes that use BBCode. With auto-url recognition and line breaks this is almost perfect for my needs. Perfect would include formatting buttons. Note that it also allows image links. (Images are so complicated in Drupal, but that's another topic...)
BBCode_wysiwyg: (Formatting buttons.) Sounded like a great addition to the BBCode module, but it's just 4 pushbuttons that don't work as the buttons on the forums. I quickly turned it off. phpBB is GPL; I wonder how hard it would be to use their formatting toolbuttons for Drupal? UPDATE: I looked at phpBB's form, and it's also just html pushbuttons. I had remembered it being fancier.
Textile: (Input format / filter, included with CivicSpace.) Cute idea. *This text bolded* _this text underlined_ ; simple markup to produce valid xhtml. I especially like the links: "link text":http://URL . Two problems: 1: it's different than what my audience is used to. 2: you have to use white space to separate the markup from other text, so a sentence ending in bold, for example, would have a space before the period. Or comma, etc. That bugs me. I converted all content to BBCode and disabled Textile.
Short note on altering input formats: While playing with input formats I wound up with existing content using several different types of input formats. I wanted to simplify choices for users to BBCode or Plain Text (yeah, it's a tad redundant), but in doing so it turns out the users can't edit content created in that input format! A node administrator has to edit each bit of content and change the input format (and formatting, if necessary) to allow the users to edit their content again. (This is only if the users can't currently use the input format the content was originally created in.) Moral of the story: Decide early in your site history what input options you want to use. Alternately offer tons of input types, but my users aren't generally tech savvy and confuse with too many choices.
tagadelic: Very nifty. It lists your categories sorted as you want, but categories with more nodes in them have a bigger font indicating more content. I've only peeked at this module a little bit. At first it seemed like it shows all terms for all vocabularies, but then I noticed it created a nested menu item with links for each vocabulary. I wasn't ready for that to be there but couldn't disable the stupid thing! I got clever and moved it to my Secondary Links menu (Drupal 4.7) which I don't display on my site.
votingapi: I just got this. It doesn't do anything on its own, but it is the power behind simple voting and latest and greatest. After upgrading to Drupal 4.7 I played with free tagging taxonomies and was disappointed to learn you can't have users (or guests) apply tags to existing content (a la rating/moderating/evaluating). I think I can work up something with votingapi to do this, though.
TinyMCE: (Formatting toolbar.) This thing and I got off to a bad start and just didn't hit it off. I turned it off pretty quickly and haven't tried it since. It reformatted my text box and removed all line/paragraph breaks in the box and in the output. I tried this as one of the first things I did with CivicSpace, and now that I know more about input formatting I may have to give this a try again as I'd really like some basic formatting buttons for my posters. UPDATE: I'm highly suspicious of an HTML editor, though...hackers could bypass the editor in a hand-crafted POST request. I'm much more comfortable using BBCode and escaping all HTML. Besides, I don't really care for WYSIWYG; I just want an easy-to-use toolbar.