[curtiss]

SMF stands for Simple Machines Forum. I think it's a dead offshoot of YaBB from years ago (as was YaBB SE, and, believe it or not, Invision Power Board).

Personally, I like YaBB absolutely the best. I am extremely active in the YaBB community, and have nothing but great things to say about it. Best of all, it's free.

Out of the remaining forum softwares, vBulletin is the only other that I'd recommend. None of the others are even worth fooling with.

Advantages of YaBB:
1) Flat text files instead of database
Why is this an advantage? For a lot of reasons. One, if something on your board goes missing, it's extremely easy to replace it from a back-up. It's also extremely easy to back-up your boards. You simply download the files from your FTP. You don't have to worry about your SQL back-up ending early (resulting in an incomplete file), or the built-in back-up not baking up things you need (phpBB only backs up certain tables in the database, so you're pretty much screwed if you need to restore something else).
2) Extremely active community
As Tom mentioned about vBulletin, the support and mod community for YaBB is extremely active and friendly. There are some truly amazing mods available for YaBB, and the devvers are always working on improving functionality and performance.
3) It's free
Unless you plan to change the filename (YaBB.pl or YaBB.cgi), or you plan to remove the copyright notice, it is completely free to use YaBB in any capacity you choose. It is essentially under the GPL (it's actually under the YPL, which is a slightly modified version of the GPL specifically written for YaBB).

Disadvantages of YaBB
1) Flat text file database
I realize that I mentioned this as an advantage, but it can occasionally be a disadvantage as well. If your server runs out of space, things start disappearing from your forums (if you make a post, your user information file gets wiped clean, so you essentially are removed from the boards until a back-up is restored). It also makes some functions like "search" slightly slower and more intensive
2) Server Usage
This is actually a big misnomer. YaBB is not really any more intensive than any other forum software. The new YaBB 2 is actually more efficient than many other forum softwares. Some of the badly written mods/hacks can cause YaBB to become a resource hog, but that will be essentially eradicated once YaBB 2 Final is released.