Here is my master list of browsers for the mac. I've seen lots of roundups claiming to mention every mac browser, which clearly don't. Except for Chromium, SeaMonkey, and the non-Songbird comedy options, all of these are installed on my computer. Let me know if you know of any I missed.
Safari - Comes with your mac. Very quick and slick, but a little lightweight in features. Currently my backup browser (for when something doesn't work right in one browser, or I don't want to deal with sessions)
Firefox - I'm pretty sure that you've heard of this. Used to be my primary browser. Very customizable.
Opera - A really nice, very slick browser that has lots of features that are only available through extensions, such as bookmark syncing across machines, and "Speed Dial", which opens up when you open a tab and gives you 9 (or more) spaces to put frequently used bookmarks. Also features things which to my knowledge are not available for Firefox, such as "turbo" mode which compresses things on the Opera servers before sending them to you when you're using a slow connection. It has previews of all your tabs when you double click the tab bar, or when you're cycling through your tabs using option-tab. There is an option to draw a line where the previous bottom of the screen was when you are hitting spacebar/page down/etc. to scroll, but hit the bottom of the page, so it doesn't scroll down a full page (good for finding your place again). Also, the themes available seem much nicer. In short, it has a lot of really nice features integrated, some of which are not available in vanilla Firefox, all while being much faster/less hard on the computer than Firefox. This is currently my main browser.
Shiira - A browser based on WebKit, the base of Safari. Used to be very nice, better than Safari and my backup browser, but hasn't had a release in a while and is falling behind (especially with Safari 4 released). Has visual tabs and such, though I'm not nearly as fond of its implementation as I am with Opera (either they're on or off, no easy situational switch or preview when switching tabs).
Demeter - A browser based on Shiira, sadly discontinued. Was my backup browser for a while, then it stopped working for some reason. Basically added bugfixes and minor refinements. Was quite nice in its day, but hasn't even been updated to the most recent version of Shiira.
Omniweb - A web browser thats been around for a bit, but used to cost money (the company decided to focus on other stuff, so they made it free). Reasonably nice, with visual tabs (this seems to be a popular thing with "alternate" web browsers). One unique feature is that it checks for updates on your bookmarks.
iCab - Has been around longer than Safari. I used it before I had OSX, and to my knowledge it is still the best option for pre-OSX macs. Brings back memories. Has been updated to be a reasonable modern browser.
Camino - A mac optimized version of Firefox. I'd use it more, but in terms of Firefox-based things, I care more about extensions than integration.
Flock - Another Firefox-based browser, this one integrates social networks. I tried it, but I found I preferred to just use the web interfaces of social networks.
Sunrise - Another webkit-based browser. This one has visual bookmarks and the ability to take full-page screenshots (what I keep it around for).
SeaMonkey - A continuance of the now discontinued Mozilla Suite, what Mozilla did before firefox. Has a browser, email client, IRC chat etc. I haven't tried this one, although I do have Mozilla Suite due to stupid compatibility issues with UTD's Moodle equivalent. I don't really have a need for a suite - I prefer independent programs.
Chromium - Early efforts at Chrome for the mac. I haven't actually tried this one; from what I hear, it's still very rough and incomplete, which is backed up by the downloads page (which I had to get the link to from an external website). I probably won't take a look until it's a bit more finished.
Stainless - Inspired by Google Chrome and its multi-processyness, this is incomplete browser with some promise. Features parallel sessions, where you can be logged into more than one account on a website at a time. Haven't used this much yet.
Comedy options:
Songbird - Based on Firefox, actually a music player, but it does have a built-in browser, which is good for downloading music (a panel at the bottom lists all the mp3s linked to by a page), but I'm not sure why you'd use it as your main web browser.
Lynx - A text-based web browser, for when you need a web browser in your terminal and don't need things like tables, pictures, or frames
W3m - Another text-based browser. Runs in emacs, for when your hot-keys just aren't confusing enough. Plus, it does tables! Wow!