My (unorthodox and somewhat rickety) note-taking gizmo uses Poisson's equation to classify (continuously) entries.

Basically the note-taking gizmo is a graph. Nodes are given conceptual masses either through pagerank or betweenness centrality (i.e. either through how many random walks or how many shortest paths cross a node). Then we calculate a potential energy (gravity potential) if we by inverting the graph laplacian (a few methods are available). Special attention is given to nodes that "float the most.

E: forgot to link to it! https://github.com/asemic-horizon/sursis/