RSPB As you would expect really, plenty of information about Blue Tits.
They still have a Springwatch section.
Also, plans to build a Nest Box and a good section called Youth for children.

BBC Science and Nature Most of the information from the BBC is sound, this is no exception, even for Blue Tits.
They are still running Breathing Places.
There's also Springwatch. Although the CBeebies Springwatch page seems to have gone, there's now the Green Balloon Club for children which you will have to take a look at and let me know what you think!

The British Trust for Ornithology, a very helpful and useful site.
They have a very good page on the main stages of the chicks' lives, from egg to fledgling. It's called the The BTO's Blue Tit Blog with good pictures.
They have a bit more on Blue Tits also and are running the Nest Box Challenge and Garden Bird Watch at the moment.

The Woodland Trust need a mention! This month they have a feature on Blue Tits.

Both the British Garden Birds web site and the Birds of Britain site might come in handy as useful sources of helpful facts and data.