The availability of additional channels will depend upon which satellite your dish is aimed at. Some dishes point at 'Astra 28.2 degree East '. There's a list of the extra channels available from this satellite here:
http://www.wildsat.com/astra.htm
Other dishes take their Sky programming from 'Eurobird 28.5 degrees East'. The additional channels carried on that satellite are here:
http://www.wildsat.com/eurobird.htm
Many of those channels are broadcast only for test purposes, so they might only be on the air for a short time. (Those lists are updated weekly). Additionally, you'll find some subscription services among the lists. Manual tuning still won't permit you to view these channels unless you have the relevant Sky subscription.
You might also be interested in a full list of all of the channels which you can receive free, without a Sky subscription, using any Sky box with the dish pointing at either satellite. (Those channels marked 'FTA' are accessible without a viewing card. Those marked 'FTV' are available with a Freesat card, which costs �20). All of the channels in this list (apart from the few 'non-EPG' channels listed at the very end) should be available from the Electronic Programme Guide but many people seem to be unaware of what's available:
http://www.wickonline.com/fta.htm
Chris