(Becoming more regular) guest writer Asri al-Baker solves the problem of a burgeoning SMS inbox on his P990i with a third party utility, SMS Diary. Here's the review, and it's worth noting that it's available for S60 and other UIQ flavours...
Read on in the full article.
Cool piece of software. Just put this on my N95 and like it a lot. Only downer is it doesn't manager MMS as well. But it does pick up and group messages to contacts who don't have country codes in their number in your addressbook, with those from the same contact, where the network prepends the number with the country code. Something that SMSThread falls over with (at the moment).
Another cool feature is the way numbers and names get hyperlinked which then provides the option to call or saving the number to your contact list