OEIS

The On-Line Encyclopedia of Integer Sequences® of Neil Sloane et al.

A Handbook of Integer Sequences, N. J. A. Sloane, Academic Press, New York and London, 1973.
The_Encyclopedia_of_Integer_Sequences, N. J. A. Sloane and S. Plouffe, Academic Press, San Diego, 1995, 587 pp. ISBN 0-12-558630-2.

B-file check, initiated by Martin Pedersen
Broken links detection and repair
Coincidences - search lists for sequences with common terms* WTM World of Terrel Trotter's Math (archive copy of the original webpage of 2004, c.f. his OEIS user page)
Engel expansion - English translation of Friedrich Engel's speech: Entwicklung der Zahlen nach Stammbrüchen. Verhandlungen der 52. Versammlung Deutscher Philologen und Schulmänner, 1913, Marburg, pp. 190-191
A220952: 0, 1, 2, 3, 4, 9, 14, 19, 18, 17, 16, 11, 12, 13, ..., 49 ... by Donald E. Knuth, Feb. 20, 2013, was unkn, with additional FASS curves: space-filling, self-avoiding, simple and self-similar curves
A131388, and A131393 with recovered Rule 2 and generalized negative-positive incrementing sequences of Clark Kimberling, with listing and program
A030707, A055187, A217760 and related sequences, listing and program for generalized cumulative counting, Clark Kimberling's problem no. 4
Triangles with interlacing rows, Clark Kimberling's problem no. 18; Perl and C programs find 1, 2, 20, 1744, 2 002 568, 42 263 042 752 triangles for the between condition

A003828, Numbers n such that n^4 is a primitive sum of 3 positive fourth powers: 422481, 2813001, 8707481, 12197457, 16003017, 16430513, 20615673, 44310257, 68711097, 117112081, 145087793, 156646737, 589845921, 638523249, 873822121, 1259768473, 1679142729, 1787882337, 1871713857
A035505, Kimberling's expulsion (shuffle) array array visualization
A070165, Collatz conjecture: OEIS/3x+1_Problem (solution proposal)
A112273: 5, 15, 365, 945 - a puzzle sequence
- my guess: 5*3^0, 5*3^1, 5*73 (or should that be 315 = 5*7*3^2 ?), 5*7*3^3
A213457 Intertwining numbers: a(8) = 1058349286
A291939: 1, 12, 19, 27, 37 - overlapping of Collatz sequences, was unkn, with 3D visualization