I know QGIS comes with its field calculator, which is not bad. It also comes with ftools which permits several operations on geometry and attributes. But we often need more control and I haven't seen any convenient tool in QGIS to perform such operations (where you can select fields you need for example or perform arithmetical operations on them while doing a spatial query).
Basically, I'd like to know if a SQL console (a kind of Mapinfo mapbasic window) is on the roadmap of QGIS and if not, if it would be hard to code it as plugin. If it exists yet, please let me know, i've missed it! ;-)