It’s Twitter Time! Free Vector Icon Set
This set contains 4 different poses of the Twitter bird. Both PNG and PSD versions has been included in the .zip file.You can use the icons for private and commercial projects, blogs and web-sites for free, without any restrictions whatsoever.

Twitter Bird
http://www.flickr.com/photos/fortrel/2034452099/
by Alexandre Lemieux

Twitter Bird Illustration and Icons
http://wefunction.com/2008/07/freebie-twitter-icons-illustration/
by Pasquale D’Silva

The downloadble file is in .ZIP format. It includes three Twitter birds .PNG graphics of 345 x 345px, 48 x 48px icon and 48 x 48px boxed bird icon.

Twitter Bird Icon
http://www.iconspedia.com/icon/twitter-bird-5-63.html
by IconTexto

Available in .PNG with various icon dimensions 256 x 256 px, 128 x 128 px, 64 x 64 px, 48 x 48 px, and 32 x 32 px. It is free for noncommercial usage, and you must include a credit for it.

Practika Icon Set
http://www.smashingmagazine.com/2008/09/23/practika-a-free-icon-set/
by DryIcons

The icons are available in various resolutions, starting by 64 x 64 px, 128 x 128 px and 256 x 256 px, in 32-bit transparency PNG. The set contains 11 high quality web icons in 32-bit transparency in PNG file format.