5 Business Ideas

Share my ISA (of dubious legality)
On the one hand there is a number of people constrained by the ISA limits but keen on investing their money in a reduced risk, tax efficient way; on the other hand, only a small number of people uses or maximises their ISA.
I see an opportunity to restore the supply-demand balance: investors could agree to "lend" money to the non-investors and maximize their ISAs every year, effectively using the ISA allowance on their behalf. The "borrower" would take a (very) small commission for his ISA allowance rental, a lot better than the nothing they would have received had their ISA allowance remained unused.
As the amount accumulates, the compounded interest may allow a renegotiation more favorable to the borrower.

Twitter Services
Services on the move in an area (e.g.: plumbers) update their status and location; interested parties subscribe to the service they need and request a visit if the professional is available and in their neighbourhood.

Rent My Garage
A garage rental marketplace: matchmaking between people with a garage and no car, and people with a car but no garage.

Paper-Holding Notebook Screen
Computer lids that also serve as page holders by either having a pressure system on the side of the screen to hold the pages in between, or having a supporting frame that slides outwards and becomes a vertical paper tray.
This would simplify tremendously the process of typing while reading from paper notes.

The Chosen 1
Unlike current online dating and matchmaking services, The Chosen 1 "game" goes beyond simple string matching.
Male and female participants are plotted on the screen as blue and pink dots resembling stars in the sky. The game challenges participants over time with questions, puzzles, activities, and preferences, some of which are introduced by the participants themselves, others are created by the system. Traits, tastes, preferences, characteristics, and relationships, can change and evolve throughout a season.
Participants can decide whether they want to attract opposites or like-minded personalities.
The Chosen 1 clusters participants creating "galaxies" and "systems" of attraction. Once certain thresholds are broken, participants within a constellation are allowed to chat online. If two participants of opposite sexes (or same sex if they so selected) are so close that they end up in a cluster of two, they are each others' Chosen 1 and given each others' contacts.
Game Over.

Because ideas are easy, doing stuff is hard. True, true.

Vaguely Related:
Business need: Digg for Consumer Electronics