What can be a good resume builder for a programmer?

Honestly - anything. If you have a project you can talk about and have a repo you can show code in, you are automatically ahead of someone without that (read: most coders). Even if that project is "dumb".

Many people will encourage submitting to open-source projects. And it's true -that's good to do! It's also hard to start with though, since you have to figure out their code base, practices, and usually the "easy" problems have been addressed so you have to take on more involved things that require more familiarity with the whole project.

So if you can support an open-source project, that's great! Do so! But if that's beyond your current free time, you can be better off doing something small. Something small but expandable. Frankly, I'm more impressed by someone that maintains a project over time than someone that makes a one-off tweak to an existing project, because I expect they learned more from the project they maintained and changed. They learned what NOT to do (probably by doing it), and that's a valuable skill.

As to what the small project does - I recommend two things:

• Something you are interested in, because maintenance will require effort, and if you don't care about it, your results and schedule will suffer.
• Something that does one thing you are interested in demonstrating. Are you doing web dev? Service development? Show a REST or GraphQL service you write or query. Are you into ML? Read from a dataset and show some results. Allow a user to interact with the model results. Is performance something you want to show off? Tackle the parsing or shuffling of a big data set. Don't try for too much at once - get something absolutely minimal that works, and add features over time.

Being a contributor to one or more open-source projects is an excellent way to show you have skills, both programming-wise and with respect to interactions with other team members, in a publicly visible way. There are literally tens of thousands of such projects, most of which have known bugs, and fixing some of those bugs is a great way to get your foot in the door. (Such projects also frequently need more or better documentation, so consider doing some of that, too.)

Getting an internship at a software company is also a good way to get started, and you'll even get paid! But there are many fewer opportunities and a fair amount of competition, so that tends to be more of an option for second- or third-year college students in a computer science or software engineering program.

Occasionally you may come across a web site for a small (perhaps one-person) local business that could use some love; if you've learned some Javascript and CSS/HTML skills, you might be able to offer your services in improving something their site. (Or perhaps they're not scaling well enough; offering to help get them set up on Amazon Web Services or Azure or whatnot might be even more appreciated.) That can be a tricky conversation to have, though (you're more or less telling the person who set it up that they didn't do a great job), so even if you spot such a site, it may come down to your people skills even more than your technical ones.

Finally, you could consider setting up your own web site with some flashy but non-trivial demo (for example, there are some very cool 3D pages built on top of three.js), or perhaps build your own Android app. Both are more oriented toward "front end" development, but that's still development, and if you manage to do something meaty enough, it could do the trick.

But contributing to one or a handful of open-source projects that interest you--not necessarily the huge ones since they may be almost as tough to crack as the job market, but the small- to mid-size ones--is probably the surest way to stand out. Take a look at Github, SourceForge, and Apache for literally gobs of options; you're guaranteed to find something interesting, probably far more than you'll know what to do with.