I have a welcome bot made (It's the same one being used in LtRoyalShrimp's chat). If you want just post your twitch name and I'll put in your chat. As Shrimp said it's made from mIRC Scripting. Does a random first time welcome when someone joins then when they leave the chat it gets recorded to a ini file. If they was to join again they get a random welcome back message. For example: welcome_bot: Thanks for tuning in again <Twitch name here> (<number of people> in chat)
If you want it in your chat I have set up a !join command. Just go to http://www.twitch.tv/welcome_bot then in the chat do !join #twitch_name_here (make sure it's all lower case and # is included. There is a little info section to read)
Edit: I released the scripts. You can download everything at the twitch link above. I well try to maintain it and upload new versions periodically but trying to maintain it by myself for 30 different chats on my laptop isn't idea for me.