This is the list of data sources I've used in the past, I'll try to keep it up to data. You can email me at carisenda+nidata@gmail.com if you have another source that's relevant.

Last updated: 9-Oct-2009

NISRA Geography

"Northern Ireland Statistics and Research Agency [NISRA], an Executive Agency within the Department of Finance and Personnel."

Contains such things as the Central Postcode Directory, Super Output Areas, Output Areas and the Irish Grid References for Northern Ireland. You can build ward Shapefiles from the Super Output Area data.

http://www.nisra.gov.uk/geography/default.asp6.htm

NINIS Data Catalogue

"Northern Ireland Neighbourhood Information Service (NINIS) provides free access to statistical and locational information relating to small areas across Northern Ireland."

Have a look at the 'all Datasets' spreadsheet, it contains a list of everything available, from hospitals by type to grammar schools by denomination.

http://www.ninis.nisra.gov.uk/mapxtreme/datacatalogue.asp

Wikipedia

Lot's of pages related to Northern Ireland some of which are also geocoded.

http://en.wikipedia.org/wiki/Category:Northern_Ireland

Geological Survey of Ireland

Bedrock geology at 1:500,000, among others. (a wee test run I did with Steve Rushe here uses this data)

http://www.dcenr.gov.ie/Spatial+Data/Geological+Survey+of+Ireland/GSI+Spatial+Data+Downloads.html

NI Planning Service

Search for planning applications online. If you ask them nice they'll send you a CD of all the applications they have available, for a fee. The format the store them in is very hard to parse reliably though. (A proof of concept map showing approvals (green) and rejections (red) for two Borough Councils over a year, ultimately the file format made the idea too costly to pursue.)

http://www.planningni.gov.uk/index/tools/applications.htm

Flickr

Geocoded photos, API including geo queries

http://www.flickr.com/services/api/

YouTube

Geocoded videos, API has location search

http://code.google.com/apis/youtube/2.0/reference.html#locationsp

Twitter

If you like mining at a high strip ratio Twitter does have location data to narrow down by. Reports of an API with better geo capabilities.

http://www.twitter.com/

GeoHub NI

Would like to be positive about this but I can't, I found it useless. Sorry guys!

https://maps.osni.gov.uk/geohublogin.aspx

Geocommons

What I could find from NI was actually repeated data from original sources already listed above (like NISRA). (Go NISRA!)

http://finder.geocommons.com/

CKAN

CKAN is a registry of open data and content packages. CKAN makes it easy to find, share and reuse open content and data, especially in ways that are machine automatable.

CKAN is the backend storing the metadata for packages/datasets on hmg.gov.uk/data

http://ckan.net

hmg.gov.uk/data

UK public sector information and data. In preview at time of writing, could find no NI data, asked on list about relevance to NI, no answer as yet.

http://data.hmg.gov.uk/