While I have a big fan of FlightMemory.com, I have recently come across an alternative that is very interesting: OpenFlights.org. It's not a 100% replacement for FlightMemory, but there are a few features which make it very competitive...
Pros:
The map is not static and the visualization is better than that of flightmemory.com. It's based on google maps which allows you to zoom in smoothly and the thickness of the lines tell us how often you fly on those routes (For example, my SEA-CPH line is massive compared to everything else.
Another big plus is that you can export your flights for safe keeping. You can export your information to an excel sheet and store it offline or upload it to another account. This is a big step up from flightmemory.com which does not offer this feature.
You can access to more ways to filter your information, and you can create trips to group individual flights in. This is a big plus for breaking up your flights into easily digestible chunks.
Lastly, you have the option of including land travel, such as trains and buses. It becomes a trip tracker, and not just a mileage tracker.