We're not sure how permanent it is, but Trillian's apparently giving out invites for their Trillian Astra client—Windows version only—almost immediately after you apply.
Back before Pidgin and Digsby were winning over your IM hearts, Trillian was the go-to IM app for multi-protocol support. Trillian Astra, the new and improved version of Trillain, has been rumored since 2006—officially qualifying it as vaporware in our book—but the invite-only beta has suddenly gotten very generous.
Just shove your info into the boxes on the signup page, hit Submit, and check your email. You should be getting an invite shortly after. (Make sure to check your spam folder if not.)
We're not sure if this loosey goosey signup procedure is indicative of the Windows IM client almost being finished, but it has been about seventeen years since they first started development