He's created one of the most useful tools in web development: Firebug. Also created the iUI (an iPhone-optimized UI) library. Of course Facebook has him on payroll now as well.
His personal website here: http://joehewitt.com
The PHP team deserves all the kudos in the world. I love PHP, and for the most part PHP has loved me. Between the four of these guys, you've got a large portion of PHP, Xdebug, APC and countless other related projects and modules. They've made web development a joy for me, when I don't have to write for lame projects that is.
Their personal websites: http://lerdorf.com/ (Rasmus), http://derickrethans.nl/ (Derick), http://netevil.org/ (Wez), http://ilia.ws/ (Ilia)
Back when I was trying to tune MySQL more, I wound up on Jeremy's site a lot. Yahoo has him on payroll, and he probably gets paid for being a general technical genius with specialties in MySQL and LAMPish development.
His personal website: http://jeremy.zawodny.com/
Steven has spearheaded a lot of great work with the memcached project. Not only that, but his insights on the mailing list have been interesting to read. Steven and his team at Facebook have made great improvements to the memcached project and have also helped with various PHP related modules (APC for instance.) Of course, it is always fun to read more information about their architecture too.
LiveJournal used to be the king. Nowadays with Facebook and Myspace it's moved down the totem pole. However, being king for a while did produce some cool stuff that Brad (and team) has been graceful enough to share. Probably the most popular is memcached, but MogileFS is pretty damn cool too. I also get a kick out of reading the history of their crazy LiveJournal server architecture.
His personal website: http://bradfitz.com/
Jan must write C and Lua in his sleep. He's behind Lighttpd, MySQL Proxy, XCache and who knows what else. He seems to be working on multiple projects all the time and making advances in them constantly. Not only creating new features out of need, but also just to see if it would work...
His personal website: http://jan.kneschke.de/
One word: jQuery. John, thank you for solving all my JavaScript needs. Cross-browser support, a simple syntax, and all the plugins available have made anything that used to be nasty DHTML a thing of the past. Now I don't flinch when I need to add some sort of client-side scripting to my pages - I just write a few lines of jQuery and move on.
His personal website: http://ejohn.org/
I'm so sick and tired of hitting F1 and F3 mistakenly when wanting to actually hit F2 (my Dell and IBM keyboards are different, not to mention actual accidental misfires...) and after realizing the simple fact that I can disable the automatic forward/back buttons on my IBM ThinkPad's keyboard, I thought to myself "Hey stupid, why don't you just remap F1 and F3 to F2 so you stop launching the help dialog and the search box all the time." I was afraid it might be a pain in the ass to find a cheap/free reliable utility, but this one actually popped up right away and seems to work great. Thank you so much Travis.
I give you KeyTweak: http://webpages.charter.net/krumsick/