Hi,
I have just created a two-lot property, one lot houses the home and some of the gardens, the other houses a three-car garage. The lot with a house is inhabited by a family, the garage lot is an empty residential lot.
Question is, what mod enables visiting empty residential lots by single Sims or families. Once i've heard that BackAlleySims made such a mod, but i can't find it anywhere.

Any help would be appreciated.

BackAlleySims has a mod that allows you to visit other sims at home, no mod exists for an empty lot, that's impossible.

So you mean that the only way to access the other part of the property is to place a Sim on the other lot, and then "visit" him whenever i'd like to use the other lot?
By the way, do you have the name of the mod, by any means?

Yes, that is what you would have to do. Jessa has a link http://thejessachannel.com/sims2modlist

I have found and installed the BackAlleySims mod "Visit other Sims" by Chris Hatch.

Just to clarify - you CAN visit unoccupied lots. I have installed two versions, the CJH_WalkToOtherSimsAL+ and CJH_VisitOtherSims-AL+ and you actually can select ALL lots in the neighbourhood, including vacant and empty ones.
I have already visited two empty, unfurnished houses. You obviously can't access build or buy mode on the lot you're visiting, but that's no problem as you can just open the lot directly from the neighbourhood screen and then rebuild/refurnish it for free.

There are basically two options, visit Sim and visit Lot, both of those are accessible via a sim "Walk to Lot" and a car.
So as you can see, it's pretty possible.

This sounds like a cool hack, I can see the purpose of being able to visit other occupied lot, for sure..this is something everyone enjoys in the Sims 3.
But could you tell me please, what is the point in being able to visit unoccupied residential lots?
Nothing comes to mind...

Learn something new everyday, I'd never heard of visiting an unowned residential lot. So now that you can, what's the purpose? Can you use the car?

On the site it says, that you can either walk to lot, take your car or a taxi. Depending on if you have Inteen or not..as only the last one is compatible with it, the taxiOnly.
I have Inteen, and i am not about to let it go. So that was useful info for me.
I might try visiting Sims, but unoccupied lots?..no.

But this does bring up some interesting questions. What are the gameplay differences between residential and community lots? What advantage might you get from visiting an unoccupied residential lot? The first things that come to mind are things you would not see: witches, charlatans, vampires, etc. What other differences are there?

Actually I don't think it would so different than an apartment building...if you were able to actually go inside other units. You could probably use it as a library or a 'free' skilling lot rather than having to skill at home until you got a hobby invite, and like Dan said it would be a plausible solution to having an expansive home that might lag your computer...IF they were still considered residential lots when you visited. And if you didn't have Freetime with the hobby lots that would enable you to create them. My question would be if you visited an occupied lot- would you have access to their funds, would you be included in their family (like a sim made selectable) or would they be autonomous?