Beethoven's only Violin Concerto (Opus 61) didn't come with a cadenza.
It was up to the performer to improvise his own cadenza in those days. Later it somehow it became the norm to perform a prepared, composed cadenza, so these were no longer improvised affairs. I think that's too bad, since improvisation never hurt jazz did it? B eventually retro-actively composed "official" cadenzas for his earlier piano concertos, but he never did one for the violin concerto. Probably because the violin concerto was not well received when it was first premiered. He did make an adaptation for piano (which is sometimes referred to as Piano Concerto 6) and composed a cadenza for that version. However it's a bit tricky to recreate that as a violin cadenza (unless you have 4 arms perhaps).
So I thought it might be interesting to compare several approaches to this cadenza question. In the below video I present "Classic Cadenzas", cadenzas which were written by such icons as Joseph Joachim (performed by Joseph Szigeti), Fritz Kreisler (Vadim Repin) and Leopold Auer (with Jascha Heifetz). The Joachim and Kreisler cadenzas are the most performed cadenzas for Opus 61.
The next video presents "Cadenzas by Violinists". When a violinist composes his/her own cadenza, it is probably closest to the original concept of a spontaneous improvisation. What we have here are cadenzas by Joshua Bell, Sayaki Shoji and Nathan Milstein.
Finally some "New Approaches". Gidon Kremer performs Alfred Schnittke's post-modern, ironic "homage" cadenza, Anke Schnittenhelm performs Sergio Cárdenas' "modern" cadenza, and Patricia Kopatchinskaja performs a transcription of the adapted piano concerto cadenza mentioned above, but with overdubbing to account for the extra violin part (in concerts, the concertmaster performs the 2nd violin part).
Actually Wolfgang Schneiderhan has made a single violin adaptation of the piano cadenza as well, which gets some play these days.
Many other cadenzas have been recorded for B's single violin concerto. Ruggiero Ricci has a record where he includes 14 cadenzas and you can program your CD player to try out each one (Beethoven, David, Vieuxtemps, Joachim (2 different versions), Laub, Wieniawski, Saint-Saens, Auer, Ysaye, Busoni, Kreisler, Milstein and Schnittke).
Personally, my favorites of the cadenzas above are Kreisler's and the Kopatchinskaja Beethoven adaptation.
For more information here is an incredibly detailed homage/website to this high-mileage Violin Concerto in D, which includes a listing of every recording and every cadenza ever performed (I think).
Leon Weintraub' Homage to Opus 61
And for some more info about modern performance practice of this work:
The Beethoven Violin Concerto in D Major, Op. 61: Some 20th Century Viewpoints