I used to knock up a massive pan of pasta sauce every once in a while and put it into freezer bags, it'd give me about 8 portions. Take one out the freezer the night before then in the morning cook the pasta, dump in the sauce, take to work and then it'd be 5 mins on Nuke at lunchtime.
Other one was to part cook a jacket potato in the morning then give it 5 mins in the microwave and add whatever toppings you prefer - I used to keep tins of baked beans and a pack of cheese at work for that very purpose.
Soup - endless varieties to make yourself.
Rice or cous-sous dishes work well in the microwave, make up double portions of whatever you happen to be eating for tea that night, the rest can go with you the next day for lunch.
My local Indian takeaway does massive portions so if I ever had a takeaway meal (usually a Sunday night), I'd eat half then, the other half would be Monday's lunch and I reheated the naan bread in the toaster at work.
It's always worth keeping a pack of pitta bread or similar at work as well, good for snacks or just buy some fillings from the supermarket/local deli as and when.