The good news if you are in the market for a home heating replacement boiler is that there is plenty of competition out there and there are lots of options.
One way to go with home heating boilers is simply to replace like with like. If you have been happy with your system but you know the boiler is living out the last few months of life, then you could simply look for a similarly powered unit that runs on the same fuel.
This makes a lot of sense - particularly if you have things like oil or propane tanks. Do not forget that tanks have a limited life span too, so do not base your decision entirely on this. If you choose to replace like with like then you should be in for a pleasant surprise. New oil fired home boilers (or gas or wood) are much more efficient than they used to be, and you will really notice the difference in your bills.
There is another option of course, and that is to use the need to change you boiler to look at different home boiler heating systems.
Natural gas home heating boilers are a great option for anyone with a potential natural gas supply since they rid you of the tank maintenance and disposal worries, and it means you will not have to sort out deliveries. It also tends to work out as the best value fuel.
You could also look at alternative heating sources - wood pellets, corn, biomass.... They can all do a great, environmentally friendly job of heating your home!