I have many LEGO sets (mostly Technic) and I would like to build a list of parts (including colours). Is there an online service that does that for me based on the set IDs?
Both BrickLink and Brickset maintains a database. Brickset pulls data directly from LEGO's servers, which means the data is not always accurate for older sets or brand new sets (when the parts are not uploaded to LEGO's servers yet). BrickLink's parts list is maintained and meticulously verified by volunteers, and it is the most accurate of all LEGO databases. This includes both old and new sets.
Go to BrickLink.com, click "Sets" on the Homepage, and type in the set number you are looking for. Click on "Item Consist Of", and it will give you a list of all the parts in the set, including parts names, IDs, colors, year of release, weight, etc.
While BrickLink is the largest online LEGO marketplace, and it is mainly meant for buying and selling LEGO, you can also use it as a database for the sets and parts you own. This is very useful if you have a large collection, and can come in handy for evaluation and insurance purposes, or if you ever decide to sell your collection (or parts of).
Brickset is also great for maintaining a database of your collection, but as mentioned above, it is not the most reliable as far as parts lists. The focus is mostly on collectors being able to catalogue the sets they own. In addition, Brickset gives you a full value of your collection based on original retail price (vs. the current market prices given by BrickLink), and has some other useful tools. I recommend using both sites, but if you only want to use one, I would suggest BrickLink. It will give you everything you need to catalog the parts you have.