I'm looking for a mod to D&D 4e which reintroduces spell-research in a balanced way. Are there any of them around, which have been play tested and seem to work without unbalancing things too far?
There are no 'rules' the specifically address how a character would 'research' a new spell in 4e.
I would simply ask the player what they were trying to achieve, what type of power or ritual it would be. What affects it would have and the like.
Next I would do a search to see if there was a similar one already in the game. I would do this to help me gauge what level it should be, and if it is balanced. Use your best judgment here in maintaining the balance of the game.
Once I determined all this I would use a skill challenge in the form of a research quest for the character to try to create this new power or ritual. They may need components or advise or books, it would be up to you to establish how hard the SC is and what would be needed. The skill challenge could be a quick set of rolls to determine if there is success and how long it takes or you could run an adventure based around the whole thing.
In addition to @Acedrummer's beautiful answer, there are rituals. Rituals are basically all the spells from previous editions that required more than a standard attack action or round to cast, with a few new goodies added in. In almost all cases, the are more powerful than the regular "power" spells that your wizards can cast, even though they don't do damage for the most part, and in order to acquire them you can have ritual uses do research and other similar things that were options to get spells in the previous editions.
As a side note for rituals: if one does not exist that does what you need, it is far easier to create a ritual than a power/spell and for the most part whatever you create will not horribly unbalance the game.