I can only offer a completely unscientific assessment of what I personally see as a freelancer in the UK, which may be completely unrepresentative (and I would be most interested in the replies of other freelancers on the forum). So, in descending order starting with the most common:
- Programming (C# and .NET especially and surprisingly little Python)
- Web mapping (Java and ASP seem most desired)
- Data management (ArcServer and Oracle Spatial most requested unsurprisingly)
- Oil & Gas geo-processing using proprietary oil-industry software and usually requiring knowledge of oil-industry work-flows and geological data.
There is then a jump down in skill level to:
- Data collection and digitization (not so much of the latter as most of this work gets sent to sweat shops in India and China for a fraction of the price. These are places where time and motion studies are down to individual mouse clicks to maintain the competitive edge.
- Mapping - ok so it's contentious listing this as a lower grade skill I'll grant you - I am liable to be lynched by cartographers but this is arguably something any GIS professional can do (irrespective of whether they do it well) and it seems to pay less than programming.
Next there are the niche jobs, which I can't possibly list as they are many and varied and I probably don't even know about half of them. This is where I get most of my income (especially 3D landscape modelling and wind farm related analysis), but that's a reflection of personal interest and skill set rather than the market per se.
Over the last 15 to 20 years I have seen a dramatic increase in the number of GIS jobs being advertised, especially in the last five years, whether contract or permanent. A lot of organisations have GIS professionals in-house now across all business sectors. What I find is that I am often providing services to companies who have access to bigger GIS firms (or firms with bigger GIS departments) but those firms are over-stretched, too generalised or I'm able to offer a skill that goes beyond the normal rank-and-file GIS professional (i.e. within my niches). Most of the contracts seem to be let by private business and less by government/councils. This may be due to the different cultures, where private business is more comfortable/used to using contractors.
The other thing I am starting to see is a slowly increasing number of requests for FOSS4G related skills. Without a doubt ESRI and MapInfo seem to be most requested, but a few years ago I would never see anybody asking for skills in QGIS, PostGIS, Mapnik, Openlayers or GeoServer etc. Now they do sometimes pop up on the radar.