Which organism has the smallest genome length?
Since you said plant/animal/anything, I offer the smallest genomes in various categories...

(Kb means Kilobases, Mb means Megabases. 1 Kb = 1000 base pairs, 1Mb = 1000Kb)


Smallest plant genome: Genlisea margaretae at 63Mb (Greilhuber et al., 2006)
Smallest animal genome: Pratylenchus coffeae (nematode worm) at 20Mb (Animal Genome Size DB)
Smallest vertebrate genome: Tetraodon nigroviridis (pufferfish) at 385Mb (Jailon et al., 2004)
Smallest eukaryote: Encephalitozoon cuniculi (microsporidian) at 2.9Mb (Vivarès &amp; Méténier, 2004)
Smallest free-living bacterial genome: Nanoarchaeum eqitans at 491Kb (Waters et al., 2003)
Smallest bacterial genome: Carsonella ruddii (endosymbiont) at 160Kb (Nakabachi et al., 2006)
Smallest genome of anything: Circovirus at 1.8Kb (only 2 proteins!!) (Chen et al., 2003)


Refs...


Chen, C.-L., Chang, P.-C., Lee, M.-S., Shien, J.-H., Ou, S.-J. &amp; Shieh, H.K. (2003) Nucleotide sequences of goose circovirus isolated in Taiwan. Avian Pathology: Journal of the W.V.P.A. 32 (2), 165–171.
Greilhuber, J., Borsch, T., Müller, K., Worberg, A., Porembski, S. &amp; Barthlott, W. (2006) Smallest Angiosperm Genomes Found in Lentibulariaceae, with Chromosomes of Bacterial Size. Plant Biology. 8 (6), 770–777.
Jaillon, O., Aury, J.-M., Brunet, F., Petit, J.-L., Stange-Thomann, N., Mauceli, E., Bouneau, L., Fischer, C., Ozouf-Costaz, C., Bernot, A., Nicaud, S., Jaffe, D., Fisher, S., Lutfalla, G., et al. (2004) Genome duplication in the teleost fish Tetraodon nigroviridis reveals the early vertebrate proto-karyotype. Nature. 431 (7011), 946–957.
Nakabachi, A., Yamashita, A., Toh, H., Ishikawa, H., Dunbar, H.E., Moran, N.A. &amp; Hattori, M. (2006) The 160-Kilobase Genome of the Bacterial Endosymbiont Carsonella. Science. 314 (5797), 267–267.
Vivarès, C.P. &amp; Méténier, G. (2004) Opportunistic Infections: Toxoplasma, Sarcocystis, and Microsporidia. In: World Class Parasites. Springer US. pp. 215–242.
Waters, E., Hohn, M.J., Ahel, I., Graham, D.E., Adams, M.D., Barnstead, M., Beeson, K.Y., Bibbs, L., Bolanos, R., Keller, M., Kretz, K., Lin, X., Mathur, E., Ni, J., et al. (2003) The genome of Nanoarchaeum equitans: insights into early archaeal evolution and derived parasitism. Proceedings of the National Academy of Sciences of the United States of America. 100 (22), 12984–12988.