CrunchSMS is a popular alternative to your traditional SMS service on your BlackBerry. It gives you an iPhone-like threaded skin and functions just like your normal SMS service, but without MMS integration.
CrunchSMS has just been updated to v2.17 with support for the BlackBerry Torch 9800 and OS 6. There is a free version that is “ad supported” as well as a paid version for $4.99, which eliminates any banners.
You can pick up the free version here.