Many Unitarian websites link to the Belief-O-Matic questionnaire. This is a questionnaire which asks about your views on life, ethics, spirituality, religion, the divine, and so on, and works out what religion you are by comparing you to other people who have answered the questionnaire in the past. One example of a church with a link to the questionnaire is Rosslyn Hill Chapel in London; another is Bristol Unitarians.
Another useful gadget is the Flickr badge, which enables you to display photos uploaded to Flickr (a photo sharing website). Of course you will need some digital photos and a Flickr account first. Once you have joined Flickr, be sure to join the UK Unitarians group and add your photos to the group pool. You can also use photos from Flickr for your website, as long as they are shared via Creative Commons.
The Unitarians in the UK & Ireland website has a map of all the Unitarian congregations in the UK, and you can zoom in to a region and see all the chapels in that area. You can also embed a chunk of Google map in your website to show where your chapel or church is. See the Bristol Unitarians 'how to find us' page for an example, and Google Help for instructions.
Who has visited
The best tool to use to find out how many people have visited your site is Google Analytics. This is not a web counter, it is a proper statistical application which is very easy to use and produces maps and graphs of your visitors. Don't use web counters - these are notoriously inaccurate, and make your site look amateur.
Another tool that is quite nice is the MyBlogLog Recent Readers widget. This shows when registered MyBlogLog users have visited your blog or wiki. I wouldn't recommend having a guestbook as these are highly vulnerable to spam.
Youtube Videos - Some congregations have created videos and slideshows that have now been put onto Youtube. You can embed video in your own website.
Another option is to have a wiki where people can post church-related discussions, activities such as engagement groups, coffee rotas etc. Cambridge Memorial Unitarian Church has a wiki (NB this is not intended to replace their main church website).
Some churches (such as Bristol Unitarians) use a blog as a "poor person's content management system"; others use Google Sites (such as New Unity). This enables multiple contributors to the website, and doesn't necessarily mean that editors need to know HTML, though it does mean they need to be confident with online editing tools (which are usually WYSIWYG).