I'd like to write a paper about a specific open-source software project that I've built but I'm puzzled whether it would be appropriate to write a paper about it. After all, not many important software projects were published in the form of a publication.
Edit: as a concrete example, this is one project I've built: http://lmatteis.github.io/void-graph/ - it can visualize RDF structures as a dynamic graph which can be saved in SVG and used in presentations or slides. Would you find this appropriate as the subject of a publication?
I can help with specific suggestions for suitable venues in your area (that accept system papers) in order of decreasing impact for improving your research rep:
Semantic Web Journal accepts Tools and Systems papers. You need to be able to demonstrate real-world impact of the tool/system. Effectively, this track is for paving the cow-paths: publishing about tools/systems that are already well-known in the community but don't have sufficient scientific contribution for a research track (and incentivising the developement of tools/systems that are useful for the community). First step is to get your tool/system to be well-known.
WWW Demo Track: Write a short 2-page paper on your idea, pack it with as much academically-restrained ehthusiasm and technical detail as you can and hopefully you'll get to present it at the WWW conference. These 2-page papers will be published in the supplementary proceedings and will be indexed in DBLP. The criteria for demo papers as WWW is (in my opinion) often fickle ... a lot of demo papers are borderline/rejected full papers. Otherwise reviewers follow their own whims.
ISWC|ESWC Demo/Poster/Challenge tracks: Probably you want to aim for a demo track. Submit a four or five page (LNCS) paper to ISWC or ESWC describing your demo. Main emphasis for reviewing is on the novelty of the system itself, technical soundness, and how nice a conversation-piece it will make at the poster/demo session. Demo papers are sometimes (not always) published as a CEUR proceedings, which will sometimes end up in DBLP.
The first option is essentially free (money wise) for you.
The latter two options will incur the cost of attending the conference to present a demo. If you are an independent researcher, that might not be an option: it might be a high cost for little reputation gain. But it depends on your long term goals.
Another option is to find your inner scientific contribution and go for a research track submission.
The most important aspect is that an expert in the area will learn something about the area that they didn't already know and couldn't find out about elsewhere (without doing the research themselves). As a reviewer in a research track, after reading a paper I will always ask myself: did I learn something? What did I learn? What is its nature (theoretical, experimental, analytical, synthesis, etc.)? Where else could I have learned that?
As an author, I apply the same principle in reverse: what is the reader going to learn from this paper and how can I highlight it and frame it in the proper "research-speak"? (This may appear cynical, and perhaps it is a little, but being able to identify, highlight and sell your core contributions is a delicate art that does lead to better papers ... as well as higher success in peer review.)
Ultimately, with experience on your side, it sometimes doesn't require much effort to find an angle from which something can be turned into a scientific contribution.
Also, take encouragement from the fact that many of the most highly cited papers/references in the Semantic Area refer to software projects or systems of various types (Google Scholar citations): Jena (856), Sesame (1346), Protege (1060), DBpedia (1344), OWL API (265), and so forth.
Being system papers, all of these papers were (arguably) arguable in terms of scientific contribution.
Likewise, many authors in the area have made their names through works that are inherently practical while being based in industry (e.g., HP Labs, Talis, Bell Labs). Looking through the author list of some of the papers above will throw up some such names.