Rodinal is a film developer developed by Agfa in the late 19th, early 20th century. It requires low dilution, 1 to 25 or 1 to 50 being normal. It also has a very good shelf life which makes it ideal for irregular amateur use.
Agfa no longer make Rodinal but Adox sell it under the original name. Other manufacturers sell the same formulation under different names.
Paranol-S is a variant I have used as well as the orignal Agfa when still available.