If you want to jazz up your computer, there is a really nice page called Star Trek In Sound and Vision that has got tonnes of images and sound files.
A great Trek site to visit, for a laugh is You Can't Do that on Star Trek. This is a great site full of images that mix Trek with other stuff, there are some nice Trek/Wars crossovers and some really cool Trek/B5 crossovers, for example seeing Q as a member of the command staff on Babylon 5!
In their own words, Memory Alpha "is a collaborative project to create the most definitive, accurate, and accessible encyclopedia and reference for everything related to Star Trek."
Another cool site is Roberto Castillo's Star Trek page, this has some really funny "top tens" and some good links too.
The Online Freedom Federation is an organisation of Star Trek fans, formed in 1997. The objective is to find an agreement with Paramount/Viacom where their copyrights on Star Trek are protected from becoming public domain and the webmasters can use their copyrighted material, with some restrictions. See the Viacom protest for more details. I've signed the petition, have you?
The Lurker's Guide to Babylon 5. contains an episode guide, plot points, analysis for each episode and has plot discussion from JMS. Possibly the most complete B5 site on the Web.
Babyon Park is an amusing satire of both Babylon 5 and South Park. It is very funny, it's from the same person who wrote "Voltayre's Enclopedia Xenobiologica", which has now been taken offline.
Visit the Official B5 home page. This is quite a good site, as well designed as you would expect from the official site (although, I do have to say that it is very "commercial", that is to say that there are a lot of adverts).
The Sadgeezer's guide to Babylon 5 is an irreverent look at B5. It contains a light look at the principle characters, the species and the ships, as well as the Babylon 5 purity test!
Now, this is a really cool site, it's Da Bearz Lair. This page is a BaBearlon 5 site with a DiFFerEnCe :-).
The Gold Channel, is a very nice site relating to Babylon 5 airing in the UK.
Excalibur: A guide to Babylon 5 - Crusade is exactly that, a good (if mis-spelt) site that contains an overview of Babylon 5 as well as news and pictures on the now discontinued Crusade.
How could I have an SF site without a reference to The Official Star Wars web site? This has all the news on the New trilogy, the Old trilogy and lots more.
If you're a Discworld fan, check out the Ankh Morpork City Watchhouse, all about the AM city watch, featuring in the Guards books (Guards! Guards!, Men at Arms, Feet of Clay, Jingo and now updated to include The Fifth Elephant).