Usability: What are some good alternative form labels for "Address line 1" & "Address line 2"?

Throughout every form you've ever filled out, online or off, it has had these two fields. They are understood by almost 100% of the literate world. Using some other names which are unexpected would surely be more confusing.

A textarea with row="2" Alternatively, if my memory serves me right, Jakob Nielsen mentioned the possibility of two text fields one above the other with a single label above the first field titled 'address'. I prefer the text area, it removes any ambiguity, but you may have specific data capture requirements that mandate two separate fields. Keep in mind that a textarea is better for int'l users who may have as many as three address lines! :)

I personally manage addresses with 3 inputs. First : name and number of the street Second : additional details like floor, building... Third : city and postal code For the third input, the user can decide to type in city OR postal code, I automatically suggest him the correct information while typing. There's a list which makes correspondance between postal codes and cities to eliminate distorted addresses.

I would say a single address line which incorporates the street address AND unit type plus unit number.  The unit type (suite/apt/unit/room) and number generally are placed in the addressline2 field.  However standardized addresses have the two fields combined into a single address line: 100 Main St #100. The other use I have heard Address2 used for is as a comment line for delivery purposes: leave on porch, around the back, north side of building, but these are really not address related in my mind and therefore should not be missed.

Why do you need two fields to begin with? I don't get confused by line 2, but to be honest every time I see this format, I pause for 1 second and wonder what if anything I should put in there, simply because now the choice is there. Should I put my apartment in line 1 or line 2? Do I put floor #? etc. Even the fact that I have to notice it is an interruption. I can think of several valid situations where you'd have 2 or more fields to get address info, but "Address Line 2" is too generic for those cases. I just don't see the point of it.

If it is an address, store it in one data field, hence use one form field. There is absolutely no reason to distribute an address over several fields. The two fields made sense in the pre-Internet era, when you needed to align rows in desktop publishing for printing out address labels. Have a look at your Google contacts and you will see there is only one field used for the address.

Depends on where your users are based. In some countries address formats diverge quite significantly from the "standard" for the US, let's say, so two address lines are pretty much essential if you're catering for international users.