Once IM support gets back into Twitter that is…
In case you don’t know Fring can let you connect to GoogleTalk, MSN, Skype and now SIP accounts all in one. It does eat a fair bit of memory and works the battery over pretty well while it’s running, but overall it’s a really nice application. If you have 3G data or wifi on your device you can make VOIP calls which is always a good thing.