Playtime allows you to download the source code of the gmail signature generator script but I personally like the styles of gizmo967.
Both these services are very simple to use: Enter you GMail username, click the 'Create' button and save the generated graphic as a .png file. Convert it other formats like .jpg or .gif using free tool like IrfanView. Done ! And putting a graphic instead of the email prevents you from spam as well.
Some more services for GMail and other popular email services.
1. http://www.nhacks.com/email/index.php - Get email graphic signatures for GMail, Hotmail, Yahoo, MSN, AOL, ATT, Bigfoot, RocketMail, QQ, Comcast, Netscape, Blueyonder, SBCGlobal, Earthlink and Lycos
2. http://www.hkwebs.net/catalog/tools/gmail/ - Only for GMail
3. http://createsigs.com/ - Generate a cool looking signature for free to use on GMail, Hotmail, MSN, Yahoo!, AOL, ATT, Bigfoot, QQ