Soprano saxophone. We have encountered concertos for two other members of the saxophone family before (alto and baritone), now it is the turn for the highest-pitched instrument of the group. Its most famous use in classical music is probably Ravel's Bolero. There are a few concertos for it, though hardly any by a composer who is at least a bit known. I have selected my favourite of them, composed by Takashi Yoshimatsu. It is played by Nobuya Sugawa and the BBC Philharmonic under Yutaka Sado, available on a Chandos CD.
 Banjo. Originating from African-American musical heritage, the instrument is now mostly associated with American folk music. Its use in classical music is obviously limited, but banjo virtuoso Bela Fleck composed a Concerto for Banjo, which was premiered in 2011, with Fleck performing with the Nashville Symphony Orchestra. As far as I know, there are no commercial recordings yet, and the reviews of the concert were not very positive - but for completion sake I include it in the list.
 Euphonium. This brass instrument is a regular feature in brass bands, but its use in classical music is limited, although it has been used as a substitute for Wagner tubas. There are a few concertos for it, though hardly any by a composer who is at least a bit known. I have selected one composed by David Gaines. It is played by Jiri Vydra and the Moravian Philharmonic under Vit Micka, available on an MMC CD.
 Harmonium. Also known as the pump organ, this instrument was widely used in smaller churches and in private homes in the 19th century, but it is not regularly seen in classical music, for the obvious reason that larger and more capable organs are at hand in concert halls and churches. I am aware of only one concerto for it, by Dutch composer Martijn Padding. It is played by Dirk Luijmes and the Asko-Schoenberg Ensemble under Etienne Siebens, available on an Etcetera CD.
 Paper. One of the most unlikely concertante "instruments" - a once-off experiment by the famous contemporary composer Tan Dun, and a pendant to his water concerto that I posted before here. Both can be viewed as special percussion instruments. This concerto is available on DVD only, played by Haruka Fujii and the Royal Stockholm Philharmonic Orchestra under Heln Elmquist.
 Electronic valve instrument. This instrument was developed by Neil Styner around 1975. Its main use so far has been in jazz/rock and new age recordings, but Maurice Jarre (best known for his movie soundtracks) composed a concerto for it. This is available on a BBC CD, in a performance by Neil Styner himself with the BBC Concert Orchestra under the composer.
 Oboe. Back to a common orchestral instrument, but still not that common in concertos, when compared to piano and violin. I have selected Bruno Maderna's second of three concertos as my example. All three are available on a Philips CD, in a performance by Heinz Holliger with the Cologne Radio Symphony Orchestra under Bertini.
 Great Highland Bagpipe. This is the instrument commonly referred to as bagpipe(s). One of the more unlikely instruments to be associated with classical music, I know of only one composer of note who used it in one of his scores: Peter Maxwell Davies in his Orkney wedding, with sunrise. We have to look to less common names to find an actual concerto for this instrument. Kevin Weed is one of them (actually the only one I found) - his 20 minutes concerto with the composer as soloist and an unnamed orchestra/conductor can be found on YouTube. Predictably perhaps, it has not reached the CD medium yet.
 Classical guitar. Its relatively low volume caused it to fall out of favour as concertante instrument in the romantic period, but 20th century composers have taken up the challenge once more. Most famous of all is of course Rodrigo's Concerto d'Aranjuez, but other notable composers of guitar concertos include Villa-Lobos, Hovhaness and Arnold. I have selected Malcolm Arnolds guitar concerto from 1959 as my example. The version I have is on an RCA CD, in a performance by Julian Bream with the Melos Ensemble under the composer.
 iPad. Surely one of the most unlikely concertante instruments. Still, composer Ned McGowan composed a substantial 25 minutes concerto for the tablet and orchestra, utilizing eight apps with graphical interfaces where one can control sound through gestures on the touchscreen (more information here). A performance of the concerto (Keiko Shichijo on the iPad with the Sinfonia Rotterdam, conducted by Conrad van Alphen) can be found on YouTube.