Peter you can get some ASTER data for free by writing to the ASTER science team, sometime they invite you to write a very bried proposal about your study and can provide you free data for that.
I'm doubtful you will be able to download the raw imagery from Wikimapia. You may however be able to download individual image snapshots which you can then mosaic together. Beyond losing quality due to compression you would have a hell of a time creating a mosaic of these, especially if you want high resolution images.
You could also try to apply for some very high resoluted images directly at commercial image providers like digital globe. If you use the data for your thesis your chances are quite good.
Besides the sensors mentioned by our colleagues, Landsat 7 & 8 provide also a 15m panchromatic band. With image fusion techniques you can achieve quite good multispectral results in thes resolution.
Free satellite imagery less than 30x30 meter is hard to come by. There is a lot of higher resolution data out there that is free for public use that is not satellite. However, it is often region specific. For example, you can get 1x1 meter digital orthorectified photography like: NAIP (National Agricultural Imagery Program) for most of the United States. There is also a lot of LiDAR and high resolution photography (20cmx20cm) near urban centers as well. Consult your local and state governments. You might look for partners in the business sector to. Often there is data out there taken for other purposes that may serve you.
A straight answer to your question is Landsat 8 and spatial enhancement or pan-sharpening using the panchromatic bands you can get a spatial resolution of 15 m or precisely 14.25 m.