So, for the benefit of one viewer (and anybody who happens to be looking for Buffy-related beta-readers) :
There's a list of S/X beta readers at http://www.jbx.com/~boethius/betas.html .
I've worked with both Firehorse and J Calanthe, and know that they're both very thorough, as well as friendly. Diana has never betaed for me, but I know her fairly well through e-mail exchanges, and from the quality of her own writing, and I'd surmise you couldn't go wrong with her either; she's a stickler for canon in canon-based stories, and an expert with Tara-voice. There are nine readers on the list, though, so if you've an S/X fic, I think there's a good chance you'll find someone willing and able.
There's also a central list for BtVS/Angel beta readers at the BtVS
Writers' Guild: http://btvswritersguild.dymphna.net/betareaders.html
If it's not Buffy-related, I'm not sure where to send you, though
I'd bet a quick Google search for "beta readers" and "fandom name here" would work!
On a side-note -- if anyone would like to be added to the S/X beta readers list, check out the list for an idea of what to tell us about yourself, then drop James and I an e-mail at email@example.com and we'll add you happily.