Good question, Ichago. Here are the tools I commonly use, in no particular order:
1. For the bulk of my building activities, I just use the viewer's built-in tool set. I've never seen any need for most of the scripted add-ons you see for sale all over the place. Despite many of those systems' authors' claims to the contrary, SL's built-in tools are more than capable of doing anything any builder will ever need, often at higher speed and with greater efficiency than what any of the scripted tools can achieve. Proper use of the on-screen rulers and manipulators, for example, completely negates the need for any of the flush-alignment, or gap-filling commands those systems often include. I do applaud the programmatic cleverness that goes into the creation of those scripted tools, but at the end of the day, 99% of them are completely unnecessary.
Where I do use scripted build tools is strictly where they can alleviate tedium. When tasks simple tasks are too repetitive to be enjoyable, or too time-consuming to be practical, is when they're better left to automation. Read on for examples.
2. I often use Cadroe Murphy's shapemaker when I need large faceted cylinders. They're not hard to build by hand, but shapemaker is usually faster, so it makes sense to let it do its thing.
3. For symmetry, I use Jeffrey Gomez's prim mirror quite often. It's not always ideal for every situation, but it can usually save me from having to stop to do math when I want to mirror a multi-prim object, so it's nice to have.
4. For texture alignment, I use two tools constantly. One is a hi-res alignment pattern I created, which I shard for free in a post on the texturing forum a few months back. I rely on that thing every day. It's so handy.
The other is a script called TexFix (can't remember the author's name off hand, sorry), which is a tremendous time saver for alignment on planar surfaces across multiple prims. As easy as alignment is conceptually, especially when it's just planar, there's no escaping the fact that doing it by hand is extremely time-consuming when there are a lot of prims involved. But with TexFix, all I have to do is set the root prim the way I want it, and two clicks later, the whole linkset is adjusted to match. It saves countless hours per project.
On non-planar surfaces, there's no choice but to do it by hand. No script can ever be clever enough to understand how a human user wants any given flat texture to wrap around a complex shape made up of lots of little components. The alignment pattern is invaluable in these situations.
5. When I need a very complex shape, especially a large one, I'll often model it in Maya first, and then use Jeffrey Gomez's OBJ Importer to replicate the shape out of triangular prims in SL. Since this obviously is extremely inefficient in terms of prim-count, I'll usually treat the resulting object simply as a scaffold, on which to build a more efficient version of the same shape by hand. Having the scaffold in place cuts down on a lot of guess-work that would otherwise be necessary in order to best visualize the shape in SL.
6. For sculpties, I use ImportPrimscript or the Qlab Assembler, depending on how SL is feeling on any particular day. Lately, one or the other always refuses to work properly. Having both is a must. (Not that you'd likely sculpt a house.)
7. Blueprints, blueprints, blueprints. I can't stress the importance of this enough. While freeform "doodling" with prims can be a great way to get your creative juices flowing, it's almost never a good idea to begin a build in earnest without first making precise diagrams of your subject. I usually make mine in Illustrator, sometimes in Photoshop, depending on the needs of the project at hand. I create front, side, and top view images of the model, and then lay them out, full size, on flattened cubes in SL, to form a 3D scaffold of 2D imagery. Then I simply build right on top of them. It's analogous to how image planes are used in more traditional 3D modeling programs.
With houses and buildings, I won't always do all three views. Usually, just a good floor plan is enough.
8. Good old fashioned brain power. 'Nuff said.
That pretty much covers it, for me. I hope it was helpful.
Thanks, though.